1. Field of the Invention
This invention relates to the electrophotographic art and more particularly, to electrophotographic photosensitive elements comprising zinc oxide as a photoconductive material.
2. Description of the Prior Art
As is well known, a number of photoconductive materials have been used in electrophotographic photosensitive elements, including amorphous selenium alloys, zinc oxide, cadmium sulfide, and organic photoconductors. Among them, zinc oxide has a number of advantages: because the oxide itself has no toxicity, photosensitive elements comprising the oxide show no toxicity; the oxide is inexpensive and can be applied by any known coating techniques; it is easy to apply the coating on large areas; images of good quality can be obtained; spectral response of photosensitivity can be suitably controlled; and the oxide is electrically chargeable and photosensitive in both negative and positive modes. In recent years, there have arisen serious problems of the effect of chemical substances on human body and of environmental pollution. This is also true for photosensitive materials. In currently employed photosensitive materials or elements including starting materials, it is only zinc oxide that is confirmed as ecologically innocuous. In this sense, zinc oxide has been recently revaluated.
Zinc oxide photosensitive elements have been heretofore fabricated by forming, on conductive support, a photoconductive layer which is made of zinc oxide powder sensitized with organic dye sensitizers and organic polymer binders.
Electrophotographic properties of zinc oxide photosensitive elements are greatly influenced by the types of zinc oxide, dye sensitizer and resin binder contained in the photoconductive layer and the mixing ratio of these constituents. The resin binder used in the zinc oxide photosensitive element usually contains therein given amounts of electron-acceptive polar groups such as carboxyl group, hydroxyl group, epoxy group, silanol group and the like. These polar groups serve to improve the dispersability and fluidity of coating paint for the photoconductive layer by interaction with the surface of zinc oxide. Once the photosensitive element is formed, the interaction between the zinc oxide surface and the polar groups contributes to control a charge acceptance and a photosensitivity. With regard to the mixing ratio of resin binder, when a mixing ratio of a resin binder to zinc oxide is too low, potential stability of a photosensitive element in repeated use and mechanical strength of the photosensitive layer are undesirably lower. On the other hand, when a mixing ratio of resin binder is too high, a practical level of photosensitivity cannot be obtained. For the reasons described above, resin binders which are employed in known zinc oxide photosensitive elements should essentially contain given amounts of electron-acceptive groups as indicated above. Moreover, an amount of resin binder is limited to a range of 10 to 40 wt % based on zinc oxide.
The vital drawback of zinc oxide photosensitive elements is that their durability is very low. Selenium photosensitive elements or cadmium sulfide photosensitive elements have a durability corresponding to 20,000 to 100,000 copies but zinc oxide photosensitive elements have a durability of as small as 500 to 2,500 copies.
The reason why the durability of zinc oxide photosensitive element is so low is considered as follows: electrical, chemical and photochemical degradations take place due to repetitions of a charging and exposing cycle; and physical and mechanical degradations occur due to repetitions of a development-transfer-cleaning cycle. The former degradation involves (1) degradation caused by corona discharging current, (2) degradation by oxidation of dye and binder with ozone, (3) degradation by oxidation dye and binder with singlet oxygen, (4) degradation by oxidation of dye and binder with photo-generated positive holes, and (5) degradation by oxidation of dye and binder with OH radicals. The latter degradations likewise involve (1) destruction of the surface of photoconductive layer by developer, transfer paper and cleaner, and (2) filming of toner. The photosensitive material or element degraded by these factors is found to involve a number of disadvantages such as a decreasing of surface potential, an increase of dark decay rate, a lowering of photosensitivity, an increase of residual potential, and a remarkable pre-exposure effect. The resulting image will have defects such as a lowering of image density, increasing of background density, a decreasing of image contrast, remaining of residual image, occurrence of white spots and a fading of photosentive element. Of these defects or factors, the durability of zinc oxide photosensitive material depend on degradation of dye sensitizer and mechanical destruction of the photosensitive material.
In order to avoid the above problem, a number of techniques for improving the repeated durability of zinc oxide photosensitive elements have been recently proposed. For instance, there are proposed the following techniques:
(1) zinc oxide particles are encapsulated with compositions comprising dye sensitizers (Japanese Laid-open Patent Application No. 54-99635);
(2) an insulative protection layer is formed on a zinc oxide photoconductive layer (Japanese Patent Publication No. 57-19780);
(3) a mixing ratio of resin binder is increased (Japanese Laid-open Patent Application No. 56-65141); and
(4) poly-N-vinylcarbazole (PVK) is used as a resin binder for zinc oxide (Japanese Laid-open Patent Application No. 56-125746).
However, all the known techniques have the respective drawbacks. In (1), capsulation conditions are very severe in order to obtain good photosensitivity. In (2), in order to form an electrostatic latent image on a photosensitive material, it is necessary to use techniques disclosed, for example, in U.S. Pat. No. 3,041,167, Japanese Patent Publication Nos. 42-19748, 42-25223, 43-1552, 47-17871 and 48-2965. This involves a complicated process copying machine. In (3), durability in repeated use is unsatisfactory. In (4), photosensitivity is unsatisfactory.
A second problem involved in zinc oxide photosensitive elements is applicability of the elements to various cleaning mechanisms and particularly to a blade cleaning apparatus. In zinc oxide photosensitive elements, it is usual that the photoconductive layer contains 10 to 50% of voids therein and has irregularities of 2-10 microns in height on the surface thereof. Accordingly, the zinc oxide photosensitive element is low in mechanical strength, so that it was difficult to apply a blade cleaning system to the element.
A third problem involved in the zinc oxide photosensitive element is its low photosensitivity. A photosensitive layer obtained by dispersing photoconductive pigments such as zinc oxide in resin binder shows a so-called induction effect in photo-induced discharge curves in which attenuation of the surface potential decay immediately after light irradiation is delayed, thus causing the sensitivity to be lowered (Electrophotography of Japan, Vol. 20, page 60 (1982)).
Zinc oxide photosensitive elements may be imparted with charge acceptance and photo-response in both positive and negative charging polarities. For instance, the following techniques are known in the art.
(1) Zinc oxide which is thermally treated in hydrogen sulfide is used (U.S. Pat. No. 3,060,134).
(2) Zinc oxide which is thermally treated in the presence of hydrogen sulfide and ammonia gas is used (Japanese Patent Publication No. 53-20856).
(3) Inorganic salts of manganese or cobalt are incorporated in photoconductive layer (Japanese Patent Publication No. 52-3303).
(4) Organic manganese compounds are incorporated in photoconductive layer (Photographic Science and Engineering, Vol. 16, page 231 (1972)).
(5) A copolymer comprising 50 to 95% of an alkyl acrylate and 1 to 5% of an organic acid having vinyl group is used as a binder (Japanese Patent Publication No. 51-16148).
In order to impart positive charge acceptance and photo-response to zinc oxide photosensitive elements, there is known the following technique.
(6) A photosensitive material is provided which comprises a two-layer structure composed of a charge-generating layer containing a sensitizing dye, and a charge transfer layer of zinc oxide powder bonded by a resin having a refractive index not smaller than 1.59 (Japanese Laid-open Patent Application No. 55-60953).
However, these known techniques (1) through (6) have, respectively, preparatory and/or characteristic problems. For instance, in (1) and (2), the treatment of zinc oxide with toxic hydrogen sulfide needs a specific apparatus. An allowance range for the treating conditions for zinc oxide is very limited in order to obtain good photosensitivity. In (3) and (4), when amounts of additives are increased so as to impart positive chargeability, photosensitivity lowers. The photosensitive material of (5) is poor in photosensitivity. In (6), formation of the two-layer structure of the charge-generating layer and the charge transfer layer by coating involves a difficulty from the manufacturing standpoint. The characteristic problem common to the known techniques is that photosensitivity of photosensitive elements is so low that their use is limited only to a so-called Electrofax.RTM. system for copying machines of the direct system. In other words, the elements cannot be used in PPC xerographic system in which repeated use is possible.